Highlights 2012

New computer simulations combining the physics of intense gravity and relativistic plasmas are shedding light on how the Universe's giant accelerators keep their beams of particles pointed.

Snapshot of a simulated black hole system. The spin axis of the black hole and the axis of the jets are aligned together into and out of the screen. The jets are blue while the white spirals show the magnetic field lines.

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Black holes, famous as the mysterious entities where gravity is so strong that light cannot escape, can paradoxically lead to the most luminous things in the Universe. This is because supermassive black holes at the centers of some galaxies pull in enormous quantities of matter and then send it flying away in enormous jets of particles and radiation. The most widely accepted paradigm for how supermassive black hole environments create the jets was proposed by KIPAC director Roger Blandford and Roman Znajek 40 years ago, in which the rotational energy of the black hole is tapped to accelerate particles that get too close. The details, however, such as how the accelerated particles remain in collimated jets which shoot out nearly aligned with the rotation axis of the black hole and perpendicular to the disk of matter accreting onto it, remain one of the greatest challenges of theoretical physics. Any coherent theory must combine general relativity, plasma physics, and particle physics.

Spinning supermassive black holes and their jets involve plasma physics and particle physics at energies and scales far beyond what can be achieved in laboratories. The details are best studied with sophisticated computer simulations where millions of particles are followed and allowed to interact according to the relevant physics of general relativity - which describes gravity in regions with intense density and curvature of space, and magnetohydrodynamics - the physics of relativistic magnetized plasmas. Recently departed KIPAC postdoc Jonathan McKinney, who is now an assistant professor at the University of Maryland, has been a leader in these so-called 3-Dimensional GRMHD simulation techniques for supermassive black hole systems. In addition to astrophysics, magnetohyrdrodynamic simulations are important in the study of many processes in relativistic plasmas, such as nuclear fusion.

In a recent work published in the journal Science, McKinney and Blandford, along with Alexander Tchekhovskoy of Princeton University, report on the results from a suite of custom GRMHD simulations where they explored the evolution of these black hole and jet systems if the jets of particles were suddenly tilted to the side, under conditions with different black hole spins and thicknesses of accreting matter. The simulations show that the systems evolved back toward the state where the jets were pointed straight up and down along the spin axis near the black hole, which is how they are seen to be in active galaxies throughout the Universe. From the simulations McKinney and colleagues were able to see that this tendency to restore the aligned jet direction is caused by the magnetic field created by the jet itself, which tends, under the influence of the black hole's spin, to wind around the jet and confine the accelerated particles to it at a certain distance awar from the black hole. Farther out from the black hole, this mechansism isn't as infallible and the jets may deviate somewhat from that alignment, as is indeed seen in many observations of active galaxy systems.

Thus, the simulations reveal what the authors call a "magneto-spin alignment" mechanism that aligns the disk and jet axes with the black hole spin axis near the black hole. The mechanism was seen to be most efficient in systems where the disk of matter falling onto the black hole was thick, which happens when the rate of matter falling in is either very low or very high. This is the case with the supermassive black hole known to be at the center of our Galaxy and also with the one at the center of the nearby active galaxy M87, so these simulations can be used to make predictions about the behavior of matter near these black holes, whose regions may be directly observed with a future radio telescope interferometer project such as the proposed Event Horizon Telescope. In fact, current radio observations of M87 have already resolved sizes less than five times the radius of its black hole.

The alignment behavior seen in the simulations also has implications for how jets behave in blazars - the distant jet systems seen in X-rays and gamma rays by instruments such as the Fermi Gamma-ray Space Telescope - and for the measurements where astronomers have attempted to discern the spin rate of the black holes in these distant objects. The magneto-spin alignment mechanism for black holes and their jets may explain many of the extreme high energy phenomena currently observed, and that will be observed, in the Universe.

This work is described in a paper in the journal Science (Science Express, Nov 15, 2012). Research at KIPAC is supported by the Department of Energy, the Kavli Foundation, the National Aeronautics and Space Administration, the National Science Foundation and Stanford University, as well as private donors. We are grateful to each of these sponsors for their continued interest and support.